As the speed of microprocessors increases, consistent with CMOS transistor feature size reductions, the required power supply voltage continues to shrink. Increased load and higher processor speed result in more severe current transients on the microprocessor power supply. For example, as a microprocessor executes instructions, particularly at a comparatively fast rate, current transients may occur that may cause noise on the power supply, and may hence induce errors.
In typical implementations of a voltage regulator, the reaction of the regulator depends on a voltage difference between a load present at the output of the regulator and a reference voltage provided to the regulator. In response to a sudden increase of a load current, the load voltage drops before the regulator reacts. Consequently, a start time of a regulator reaction or adjustment never matches the start time of a change of a load present at the output of the regulator. This makes it in general rather difficult to prevent a narrow but possibly deep voltage dips on the load.
Solutions to this issue presented so far focus on an increase of the speed of a feedback loop of a regulator. Such solutions are quite power consuming and have therefore detrimental effects on battery lifetime. Moreover, even large capacitors in parallel with a load circuit may be of limited effect as such capacitors do not remove the fundamental need for a voltage difference to develop before the regulator circuit can react. Moreover, all technical solutions known so far have theoretical speed limitations.
Moreover, there is a growing number of low power or ultra-low power applications with strong emphasis on battery lifetime, which are normally operated at very low supply currents they are optimized for. In order to be of practical use, these applications and devices must also at least temporarily support high current modes where they receive commands or return data via external interfaces.
Moreover, one of main targets of electronic system design is competitive cost, leading to a strict requirement of minimum number of discrete components. In many systems in general and in portable devices in particular, regulator circuits cannot take advantage of external capacitors. Under such conditions, the integrated regulator circuit must provide a very fast and strong reaction to a sudden load change.
It is therefore an object of the present disclosure to provide an ultra-low power voltage regulator for driving a digital circuit that exhibits a rather fast, almost instantaneous reaction to varying loads. The voltage regulator should be simple and easy to implement. The implementation should be cost efficient an inexpensive and should enable fast and reliable regulation of a driving voltage for a digital circuit over a large range of supply currents.
The U.S. Pat. No. 6,177,785 B1 describes a voltage regulator for driving a switching circuit. The voltage regulator includes a programmable output adjustor, an error amplifier, and an output driver. The programmable reference generator is responsive to a first programming signal and generates a reference voltage. The programmable output adjustor is responsive to a second programming signal and generates an output adjust voltage. The error amplifier generates an error voltage corresponding to a difference between the reference voltage and the output adjust voltage. The output driver drives a regulated output voltage in response to the error voltage. However it is not defined two operating modes to be selected for preventing any narrow and deep voltage dips on a load at the output terminal of the voltage regulator, which is a drawback.